Citrate toxicity limits the volume of blood that can be processed during peripheral blood stem cell (PBSC) collections and other large volume leukapheresis (LVL) pro-cedures. This limits the number of cells that can be collected for hematopoietic transplantation. Knowledge of physiologic citrate and calcium responses at citrate infusion rates used during LVL could increase both donor safety and cell yields. Healthy allogeneic donors donating PBSC products for clinical transplant protocols underwent extensive monitoring of citrate effects and response to intravenous calcium replacement. Twenty-one donors underwent 37 LVL procedures, during which 12 to 15 liters of blood were processed. Serial pre, mid, and post apheresis labs were drawn, including blood citrate levels, ionized Ca and Mg, parathyroid hormone (PTH), and pre and post urine chemistries. Citrate symptoms were scored as barely noticeable (1), irritating (2), uncomfort-able (3), or unbearable (4). On first LVL, 18 donors (group A) had citrate infusion rates of 1.0 to 1.6 mg/kg/min, with calcium infused only for symptoms more than grade 2; three donors (group B) had citrate infusion rates of 1.6 to 1.8 mg/kg/min and received prophylactic calcium (0.45 to 0.6 mg calcium per ml ACDA). Repeat LVL (16), (ten as in A, six as in B) was done in nine subjects. In these repeat donors, replacement solution was randomized between equimolar infusions of calcium chloride (CaCl) or calcium gluconate (CaG). Donors in group A had no symptoms at citrate infusion rates of less than 1.1; at citrate infusion rates of more than 1.2, six of 12 had symptoms: three had grade 1 and three had grade 2. Donors in group B, given prophylactic calcium, had no symptoms. The mean nadir decrease in ionized calcium was 30 percent in group A and 11 percent in Group B at comparable citrate infusions rates of 1.6 mg/kg/min. Ionized calcium and magnesium concentrations inversely followed citrate levels. Marked decreases (20 to 40 percent) in ionized magnesium occurred by 30 minutes into each procedure, continued to decrease during calcium replacement therapy, but were unrelated to development of symptoms. PTH peaked at 30 minutes, then subsided despite persistent hypocalcemia. Citrate levels were consistent within donors on repeat LVL, but varied widely between donors. Post/pre urine calcium and magnesium ratios were increased at citrate infusion rates more than 1.2 mg/kg/min. Equimolar infusions of either calcium chloride or calcium gluconate had similar efficacy. These studies show that citrate levels are the main determinant of ionized calcium and magnesium levels during LVL. Citrate not only complexes blood calcium and magnesium ions, but significantly increases urinary calcium and mag-nesium excretion. Prophylactic calcium infusions according to a preset algorithm were safe, prevented symptoms of citrate toxicity, and allowed higher blood processing rates. Larger volumes of blood could be processed in shorter times, increasing the cell yield available for transplantation.